The objectives of this inquiry are to: (i) investigate the sociodemographic correlates of current STI/HIV status and history; (ii) examine the multiple ways that sexual and protective practices influence STI risk; (iii) explore how interpersonal and sexual relationship-specific factors affect these practices; and (iv) assess the ways in which social environments influence individual risk. To attain these objectives, we will analyze data from the National Longitudinal Study of Adolescent Health (Add Health), a three-wave panel survey of a nationally representative sample of over 20,000 adolescents followed through their young adult years. Several sources of data will be used in the proposed research: biomarker indicators of gonorrhea, Chlamydia, trichomoniasis, and HIV; self-reported STI histories; person- and relationship-specific sexual and protective practices; psychosocial and sociodemographic measures; and social and demographic variables for census tracts, counties, and states. The core premise of the investigation is that multiple levels of social context shape individuals' attitudes and beliefs, which in turn affect individuals' relationship experiences and sexual and protective practices, which ultimately affect their risk of STIs. The proposed study has three major components. First, we will compare biomarker and self-report data to assess STI risk. Second, we will assess the relative contributions of four critical sexual and protective practices on STI risk. Third, we will examine the effects of relationship-specific characteristics on the propensity to practice protective behaviors within relationships. The project will use several methods of multilevel modeling (e.g., fixed effects, Generalized Estimating Equations (GEE), random coefficients, classical estimation with covariance matrix adjustments via the "sandwich estimator" tailored for clustering) to account for clustering, and will also explore the effectiveness of alternate definitions of contexts in a study of STI and HIV. Because the study will use a population-based approach that incorporates multiple social contexts, it will be possible to assess the impact of intervention strategies targeted at individuals, relationships, and communities.